1. Field of the Invention
The present invention relates to chip carriers and, more particularly, to high density chip carriers with high wireability for use with flip chip technology, and the like.
2. Background and Related Art
As the terminal density of semiconductor chips and, particularly, the density of Input/Output (I/O) connections of chips increases with improved technology, the wireability of chip carriers becomes more problematic. The density of terminals tightly clustered makes it difficult to construct mutually segregated conductors to connect carrier lines to each terminal. Signal carrying terminals and lines are particularly burdensome since they must be segregated from each other as well as from power and ground lines. Signal lines on the chip carrier must have sufficient electrical isolation from other conductors so that undesired coupling and leakage paths are avoided.
To enable routing in highly dense chip carriers, microvia, as well as other technologies, have been developed. Microvia chip carriers typically use multiple layers to make the required interconnections, particularly in chip packages using flip-chip ball grid array (BGA) technology. In these high pin count technologies, the density of wiring and the wireability of the layers is important, particularly in terms of cost, yield, performance and reliability. “Wireability”, in this regard, can be viewed as the technical possibility of positioning routing lines so that all signals may “escape” (inward or outward) from a given pattern or layer. Constraint considerations for routing include via density, routing line widths and clearances, the terminal sizes and required clearances, the shielding requirements and other design constraints known in the art.
Microvia chip carrier substrates are generally built around a core with plated thru holes (PTHs). Such high density interconnect (HDI) chip carriers use build-up of layers on each side of a core made of epoxy-glass layers. The glass layers are made of a glass cloth impregnated with epoxy and are laminated at elevated temperatures to make a solid, dimentially stable core. The build up layers on each side of the core are generally non-reinforced epoxy. U.S. Pat. No. 6,518,516B2 describes a typical microvia chip carrier.
The density constraints of the PTH's in the core limits the vertical interconnection capability between the front and back of the carrier. For example, high density PTH arrangements can result in reliability failures along glass fibers, from one hole to another, when the holes are placed too close together. The inability to interconnect a large number of signals from the front and back of a chip carrier because of PTH density constraints caused by reliability problems when PTHs are placed too close together, limits the ability to connect higher I/O count chips to a chip carrier or to interconnect such chips to a printed circuit board (PCB).
Shorting between PTHs in glass reinforced epoxy carriers has been attributed to the fact that the epoxy bond to glass fibers is fairly poor. When PTHs are drilled in the core and plating chemicals are used to plate the PTHs, the poor bonding of epoxy to the glass fibers allows the plating chemicals to penetrate some distance along the fibers. This penetration can result in electrical shorting between holes.